1. Field of the Invention
The present invention relates generally to gas turbine engine, and more specifically to a turbine stator vane with a trailing edge to endwall construction and cooling.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
The trailing edge section of an airfoil is very thin compared to other sections. In a large frame heavy duty industrial engine stator vane, the airfoil extends between an outer diameter endwall and an inner diameter endwall. A fillet forms a transition from the airfoil to the endwall. The trailing edge section of the airfoil is much more difficult to provide cooling than the immediate surfaces of the endwall due to the thin section of the airfoil. It is very difficult to provide for cooling passages within this very thin airfoil section. Thus, cracks occur due to the thermal stresses induced by the temperature differences between the vane airfoil trailing edge thin corner and the relatively thick endwall. This crack formation is especially pronounced in vane segments having two airfoils per segment because of circumferential distortion and axial bow of the endwall that produces additional loading at the vane trailing edge and endwall transition location.
FIG. 1 shows a prior art turbine stator vane with two airfoils 11 formed in a segment between an outer diameter (OD) endwall 14 and an inner diameter (ID) endwall 15 with a fillet 12 forming a smooth transition between the airfoil and the respective endwall. The ID endwall 15 includes a front rail 17 and an aft rail 16 extending from the bottom end with a seal slot 18 on the ends to receive a seal between adjacent vane endwalls. Drilling cooling holes at the vane trailing edge section to provide convection cooling will produce unacceptable stress levels around the cooling holes, especially at the highly loaded locations.